Let's take a look at the example. Below is a form that contains two combo boxes. One is called CategoryID and the second is called ProductID. When a value is selected in the CategoryID combo box, the ProductID combo box repopulates its list of items based on the CategoryID chosen.
The trick to properly populating the second combo box called ProductID is how the RowSource is defined. The RowSource for the ProductID combo box is based on a UNION query that lists the records from the Product table as well as a row that contains null values. The purpose of the null value row is to clear the selection in the ProductID combo box.
